Control mechanisms for adjusting the transmission ratio of the hydrostatic transmission by means of a pump and an auxiliary pump driven by a motor are known. It is also known to employ the pressure drop caused by a throttle element arranged in the ducts of the system to use for actuating a signal for varying the transmission ratio.
In these prior art devices the throttle element consists of a regular throttle which is adjustable by the operator's action and which serves to adjust the magnitude of the phase displacement between the increase of the rotation number of the motor and the increase of the pressure in the pressure liquid of the control circuit. The adjustment is effected by the accelerating pedal of the motor. The clear fluid cross section is determined alone by the adjustment made and it determines the time period for the pressure build-up. This has the disadvantage that the throttle must be continuously or repeatedly adjusted in order to obtain an economical and reasonably quiet drive performance.